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Surface strain regulation for stretchable electronics

Author

Liu, Zhiyuan

Date of Issue

2017-11-01

School

School of Materials Science and Engineering

Abstract

Stretchable electronics, such as stretchable electrodes and stretchable strain sensors, is vital for the soft electronics, such as human-like robots with artificial skins, wearable and implantable devices, and bionic sensory systems. However, how to achieve high stretchability, sensitivity, stability and adhesion by using simple fabrication procedures, is still a big challenge. In this thesis, a new strategy, surface strain regulation, is proposed to fabricate stretchable electrodes and strain sensors with high performance and accessible methods. The principle is that if the distribution of surface strain energy could be changed in the active material into a random style but not the concentration mode, it will turn into the network instead of throughout breaking at the strain concentration point. Based on it, more than five new material fabrication methods are developed: three of them for achieving stretchable active materials, and two for new sensors development and performance enhancement.
In detail, carbon nanotubes network by coffee ring effect, nanopiles interlocking, and dynamic interface mingling are the new methods proposed to achieve stretchable conductors. Hair-like and fiber-shaped structures are for new kinds of sensors and enhancing the performance. And they are all based on this new strategy/concept, surface strain regulation. The stretchable carbon nanotube thin film is successfully fabricated by coffee ring effect benefiting the high-gauge-factor stretchable strain sensors. For this stretchable conductor, the surface strain regulation is achieved by the carbon nanotube network structure. The film with gradient thickness successfully solved the problem of combining the contradictory properties of brittleness and stretchability achieving both high gauge factor and high stretchability. Also, inspired by the piling process in the building, a transitional layer from the soft substrate to the rigid active material is constructed to achieve the surface strain regulation by the new nanopiles interlocking structure. It is a new method to fabricate stretchable electrodes meanwhile addressing the solution for the adhesion problem between the active materials and substrate, which is the big obstacle for the real application of stretchable strain sensors. Moreover, a more accessible method by dynamic interface mingling of elastic polymer and metal is reported here to fabricate stretchable conductors with high stretchability and stability, high surface area and most importantly, high adhesion between the metal, gold, and elastic polymer, PDMS in a large scale. The underlying knack is to use uncured PDMS instead of fully cured one to receive the gold atoms generated by the thermal evaporation. It will trigger a series of dynamic chemical and physical processes fabricating stretchable conductors with the unique microstructure of self-stripped kalst caves-like double layers. The self-stripped kalst caves-like layer can tune the strain distribution on top and achieve the superior performance. Further, a new kind of stretchable strain sensors, stretchable hair-like sensors with a design of gradient transition, has been proposed and produced by utilizing 3D printing and out-of-plane self-pinning effect, which will open up for exploration of stretchable hair-like sensors for soft electronic applications. By surface strain regulation of the hair-like structure, the sensitivity is enhanced and more importantly, the strain direction can be detected. Besides, a method to fabricate large-scale PDMS fibers is also proposed, by modifying the unique beads structure onto the fiber, the surface strain is redistributed and the sensitivity of the fiber-shaped stretchable strain sensors is significantly improved. All above news methods developed have proven the power of the new strategy proposed here, surface strain regulation for stretchable electronics. Based on it, many other methods could come up to fabricate new stretchable conductors and sensors.